Qt Designer vs Qt Quick Designer vs Qt Creator?

I will explain to you the difference between these tools by the approach for what they are used:

Qt Designer: Sub tool used to create/edit widget files (.ui). You can use it to create the graphical layouts (.ui files only). The most use is to design the graphical stuff in PyQt apps. It is installed always when you install Qt, for example it is in the path: Qt5.13.1\5.13.1\mingw73_64\bin\designer.exe. It also be used to edit any .ui file of a Qt C++ application, however it is very limited since only allows to edit the graphical stuff (not C++ logic).

Qt Quick Designer (it refers to Qt Creator): It does not exist, it is integrated in Qt Creator (see below). Is normal to say that Qt Quick Designer allows to edit QML files (.qml), however it is integrated in Qt Creator now.

Qt Creator: This is the so defacto and most powerfull IDE to create QT applications natively (C++ with Qt engine). It allows you to create, edit source code, debug applications, etc. In addition to that, yo can open a .ui file or a .qml file in Qt Creator and it will open and allow you to edit. For example if you open an .ui file it will show you the Qt Designer app embedded in the full integrated Qt Creator IDE. In summary, you can use Qt Creator to open/edit any .ui or .qml file and create Qt/C++ applications. Of course, if the file is .ui then Qt Creator will show you the Qt Designer tool, if it is .qml then it will allow you to edit the QML.

Qt Creator is Qt's IDE. You don't have to use it, but it greatly simplifies Qt development.

Qt Designer is a graphical tool that lets you build QWidget GUIs. Qt Quick Designer is similar, but for building QML GUIs. Both are built in to Qt Creator.

This is explained in a little more detail over at Wikipedia.